Packaging hot melt adhesives generally comprise thermoplastic compositions, typically fabricated from a mixture of polymeric resins, tackifier resins and waxes. Hot melt adhesive compositions are relatively solid and non-tacky at room temperature; however, when raised to an elevated temperature they soften and melt so as to provide a tacky, relatively fluid material. Hot melt adhesive formulations have what is termed an “open time” which is the period of time during which the heated composition remains fluid enough to allow for the effectuation of an adhesive bond. In use, hot melt adhesives are raised to a working temperature, and applied to articles to be joined. The articles are maintained in contact during the open time of the adhesive. Upon cooling, the hot melt adhesive reverts to its solid form and provides an adhesive bond between articles which it joins. Since the hot melt adhesive is a thermoplastic material, the bond can be broken by reheating the adhesive back to a temperature above its softening temperature.
Hot melt adhesives are in very widespread use; and one particular application for hot melt adhesives is for the sealing of packages, particularly corrugated and paper board packages used for consumer and industrial goods. Hot melt adhesive materials may be pre-applied to various of the closure portions of packages at the time they are fabricated. This avoids the expense and labor associated with maintaining adhesive application equipment at package filling facilities. The non-tacky and non-blocking nature of the hot melt adhesives at room temperature allows for such uses. After a package is filled, a simple heat sealing step is employed to close the package.
While hot melt adhesive techniques are in widespread use, there are problems associated therewith, and these problems are inherent to the nature of hot melt adhesives. Since the bond formed by hot melt adhesives is thermoplastic, temperature extremes encountered during shipping, storage and use can cause failure of the adhesive bond. For example, goods are often shipped by truck and/or stored in uninsulated warehouses, and can be subjected to very high ambient temperatures, and these temperatures can be sufficient to soften hot melt adhesive formulations to a degree sufficient to result in bond failure. While it is possible to raise the softening point of hot melt adhesives by varying the formulation, doing so engenders other problems, since the packages are also exposed to very low temperature conditions during shipping and storage, particularly if those packages are used for foods which must be kept cool. Low temperatures can embrittle the adhesive bond leading to mechanical failure. Problems of low temperature failure can be accommodated by softening the hot melt formulations so that they will be less brittle; however, this results in a loss of high temperature strength and also tends to make the adhesive formulations tacky at ambient temperatures. Such tackiness can cause premature adhesive bonding, and may cause packages to stick together during filling, shipping and handling; this phenomenon is called “blocking.”
Accordingly, there is a need for adhesive formulations which retain the advantages of conventional hot melt adhesives, but provide a bond which is secure over a very wide temperature range. Such adhesive formulations must be capable of being pre-applied to members to be joined, and should be non-tacky and non-blocking under ambient storage and use conditions, it being understood that in the context of this disclosure, the terms “ambient” and “room temperature” mean temperatures typically encountered in a workplace or other facility. Room temperature is typically understood to be in the general range of 10° C.-32° C., and more typically 17° C.-27° C. It is further desirable that such adhesive formulation be simple and easy to use, and it is most preferable that it be compatible with presently employed hot melt systems and techniques.
As will be explained hereinbelow, the present invention is directed to a two part hot melt adhesive composition which, upon application, undergoes a cross linking or other such curing step which further strengthens the adhesive bond. The adhesives of the present invention combine the qualities of hot melt adhesives and two component, curable adhesives.
The prior art has formulated various curable hot melt adhesive systems; but none are a two part system like that of the present invention, and none secure the benefits thereof. For example, U.S. Pat. Nos. 5,849,832 and 6,121,354 both show hot melt adhesive compositions which include a moisture activated curing adhesive component therein. These compositions are provided as a one component adhesive mixture, which is applied to surfaces at an elevated temperature and sets up to provide a thermoplastic bond. Contact with atmospheric moisture subsequently causes cross linking and curing of the adhesive which further strengthens the bond. Systems of this type would not be applicable for package sealing applications wherein the adhesive components are pre-applied, since the adhesive would then undergo further curing prior to the time that final bonding needs to be accomplished. Another approach is shown in U.S. Pat. No. 6,294,597 which discloses a hot melt adhesive which includes a thermally curing composition therein. The hot melt adhesive of this patent is applied at a first, relatively low temperature to form a thermoplastic bond, and this bond is subsequently exposed to higher temperature conditions to initiate a curing step. Adhesive materials of this type could be pre-applied; but the necessity of implementing a second, high temperature curing step precludes the efficient use of such compositions in a package sealing application.
As will be discussed further hereinbelow, the present invention provides a simple to use hot melt adhesive which forms a very strong bond which is stable at both high and low temperatures. The adhesive of the present invention can be used in presently employed package sealing technologies with minimal, if any, modification of equipment or processes.